The Enterprise is in Earth's orbit in the year 1968 for historical research, when the ship intercepts a transporter beam and a man named Gary Seven materializes on the ship. He claims to be a human being, sent by powerful aliens on a mission to save Earth from nuclear destruction. Gary Seven is taken into custody but escapes to his office on Earth, located in a building in New York. Kirk and Spock find his office, but Seven is already on his way to the McKinley base to sabotage the launch of a nuclear weapons platform. Kirk and Spock follow him but are arrested by the base's security. Scotty almost succeeds in beaming him up again before he can finish his work, but thanks to the unsuspecting secretary Roberta Lincoln, who activates his transporting device, Seven materializes in his office again. When Lincoln realizes that Seven has brought the rocket off course and activated its warhead, she tries to stop the agent. Kirk and Spock escape from McKinley and materialize in the office again. They have to trust Gary Seven that he is going to detonate the warhead in time before it can do any damage, but not without being a warning to humanity. The weapon goes off 104 miles above the ground, exactly as it was meant to happen according to historical records.

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